The correct option is B cos−1(−3132)
Let the vectors be →A & →B. Then,
R=√A2+B2+2ABcosθ
R2=A2+B2+2ABcosθ
Given that A=B & R=A4
Putting the values of A,B and R,
(A4)2=A2+A2+2×A×Acosθ
⇒116=2(1+cosθ)
⇒1+cosθ=132
⇒cosθ=132−1=−3132
⇒θ=cos−1(−3132)
Hence option B is the correct answer